This is why we value the Halbach array.
Normally a magnet creates a field that is like this:
...and if you trace the path of the flux lines they need to go in
WIDE CIRCLES.
The Halbach array effectively "short circuits" the flux paths and reroutes them back outward on just one side. This is the essential reason that the Halbach is of any value because it eliminates the big looping flux paths.
Big looping flux paths normally need to be dealt with using lot's of iron everywhere. You needed iron in the rotor and iron in the stator. Basically the standard motor designs were just one massive chunk of iron channeling flux paths around in big wide circles. All that channeling of flux paths would produce losses. (hysteresis, eddy currents, I2R losses, and saturation)
The axial (pancake) shape is not the only type of Halbach motor possible. You can also build them facing inwards and get a motor that would be of a more standardized shape:
...so it's not about "shape" but the lack of iron that we are interested in the Halbach design. We are left with just the copper wires and as far as losses all we have is the
copper loss. So if you use plenty of copper in your design (Litz) then you (by definition) will have high efficiency. These motors tend to be in the 90% and up range as far as overall efficiency.
Just want to make sure you are following what it really of value here.
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Note: My parts arrived yesterday, so I'll be starting the project thread soon.